Well I thought I would share an update. I bought an RO filter and I thought this was going to be the solution. I started watering with ro water right at the flip cause that's when I received the filter and the small mag problem I had cleared up. I was really excited and thought I found the solution to my problem. Then the dreaded started happening at week three. Here comes the mag issue again. So now I'm back to being frustrated and don't know what the problem is. It's magnesium every single time and it doesn't matter what mix i run. I even seen it when I ran coots mix one time. I know there is mag in my worm castings the maxi crop and the dolomite lime I used. Maybe the plants still want more idk. I still haven't messed with ph that's my next step but honestly I don't see how if it was ph I would only be having a mag issue and nothing else. I don't really think that's it. I'm going to work on it though. If anyone has any other ideas I would appreciate it

Ok, theory time:
I would say it is theoretically possible that just Mag gets locked out by your PH values.
If that is actually the case... There is a thread on deficiencies here on the board which also mentions at what PH what nutrients get locked out. You could theoretically consult that thread.

Anyhow:
If you think it is still Mag that is missing, why not just add some more then? You can water it in, you can topdress it and see what happens. At worst you burn your plants and have to work in the opposite direction or, absolutely worst case, reset entirely.
Which imho isn't much worse than a bad harvest.

The yellowing on your leafs, to me, could also be a N deficiency. Maybe try give them some more of that and see how they react.

I can tell you that on my end, I was searching deficiencies for almost a year, preceded by trying to figure out how I got fungus gnats that killed everything.

Ultimately I gave up and did a complete reset, including 2 months of downtime (no growing at all).
I didn't even adjust the soil mix drastically but I did create the hypothesis that it was the additional bio canna nutes I used that caused the root rot and fungus gnat explosion and that was my main issue.
I also theorized that likely not the nutes themselves were the culprit but what they did with the water I gave to the plants, meaning the PPM and PH were all out of whack from using the additional nutes.

So I decided to use only water for this run, after resetting everything. And PH the water for the first time.

And looky here, the plants look great, everything worked awesome.

What I noticed was (similar to you?) that there must still be a deficiency in the soil because I had yellowing leafs starting from the bottom and bottom growth being stunted and tiny before yellowing, dying off and then trying to grow again.

I thought about it, went online and asked questions etc. etc. and ended up narrowing it in to a N-deficiency.
So I simply tried it out.
I topdressed with a gentle helping of EWC and sure enough, the plants looked way better in flower and the yellowing from the bottom and tiny/stunted growth from the bottom of the plants stopped entirely for several weeks.
Now, a lot later in flower, they are appearing again, I have yellowing here and there and some of the plants reacted with burned tips and clawed leafs (very few) so they probably got a bit too much N from the top dress. But most plants really liked that step.

My plants in veg however, were still in the same soil mix as before. The one I since knew was N deficient.
Unfortunately I didn't have any EWC anymore to remedy this so I used bloodmeal instead. Since I didn't want to topdress pure bloodmeal (the smell ...), I took a bit of the new soil mix and enriched it with even more bloodmeal and topdressed that.

The plants loved it and the current veglings are among the healthiest I have had so far.

So:

My advice? Simply go with it and try it.
If you agonize too long over it without actually trying something, you will get nowhere and just loose time.

Do something, even if it risks losing your current crop. At least you will know you need to work in another direction then.
But nothing is more frustrating than waiting for an answer and seeing no change when applying the various solutions mentioned by people online.

It's your garden, they can't really help you in the end. They can inspire you to find the solution, they can nudge you in the "right" direction but in the end, you will have to find out.

In case of my fungus gnat infestation:
Only after about half a year of tips and tricks and suggestions of how to handle them, discussions if they were root aphids after all etc. etc. etc.
Only then did someone mention, as a sidenote, in passing, that it might be from the whole synthetic nutes in supersoil conundrum.

That still wasn't exactly the answer but it steered me in the right direction, the direction where I question my water, my PH values, consider that nutes are locked out and roots beginning to rot as a result of nutelock in constantly damp/wet soil.
I believe I was basically watering them and the water would just sit there because it was too salty/acidic/basic/whatever for the roots/plants to do anything with. Since they were surrounded by soil and water but couldn't drink any of it, they just withered and died or some of them barely pulled through with bad results.

But I had to figure that out myself in the end.

And I am still not 100% sure I have actually figured it out or just got lucky so far in the second round.

I have around 9% saturation of mag with 85% ca with no issues, watering in Epsom week 3 and week 5 at 1 tsp per gallon. My pH is around 6.4 and no dolomite lime added. A couple things I could think of, soil is too wet and your roots are having problems with uptake, roots could be too cold as well. Could also have to do with your P levels locking it out or potassium levels being too high blocking it out. If you had been watering with the old water in veg and only switched to ro at start of flowering you could have a build up of whatever it is in your soil from old water causing the problem. The best way is to test your soil and see what's there, in the mean time I would drench in some Epsom salt next watering and try to go a bit drier on your watering.

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I watered two with go calmag. I'm watching for signs of improvement. Then I got three others I'm going to give a good top dress of worm castings and compost when they need to be watered again. I let them dry out a good amount. I lift the pot to see if there ready for water. Ya I've heard the high phosphorus can lock out mag but I also heard it would lock out other micro nutrients first. I did switch water at begining of flower. I had a small mag issue then and I didn't add any nutrients but when I switched the water the mag cleared up all on it's own. That's why I was thinking the ro water would fix everything but then it returned

Thats not a Mag issue, thats a N issue. Your plant is suffering from low levels of Nitrogen. Its very common to experience this problem after 3-4 weeks into flowering if you dont continue to supply enough N, and you say this happens consistently at 3-4 weeks in??? Thats not surprising because the very same thing happens in my garden too if I fail to feed my plants enough N.

I dont know how close you keep your plants to the lights, but if you can keep your lights further away, you'll experience N deficiency setting in later in flower, rather than earlier. It may seem odd to move the light further away, but keeping fan leaves healthy and green is the priority #1. It takes some experimenting to find a good balance, but it does help alleviate N deficiency symptoms.

I fixed this with top dress of worm castings mixed with a little dolomite, kelp etc...
The chlorosis was pretty much gone in a week or so. This is also with a fairly large soil volume in a bed albeit inside.

I'll get some better pics tomorrow. It looks like the classic mag look at first. After adding go calmag to the two plants in the back the problem has seemed to stop so either way I'm not giving them enough nutrients. I added cal mag to all the pots now. It could be nitrogen but I feed them every watering.

Added to 5 gallons ro water bubbled for 24-48 hours. Then given to plants as is with no dilution through entire flower cycle. I did run out of the Peruvian guano and missed two watering with it so maybe that was enough for the plants to suffer.